Autoimmune (Idiopathic) Thrombocytopenic Purpura (ITP) is a disease caused by autoantibodies directed against platelets. Their demonstration has been difficult for a variety of reasons. In general, when the antibodies can be demonstrated, there is an inverse correlation with the platelet count. We have set up two assays, one screening assay and one specific assay for specific platelet glycoproteins, to aid in the diagnosis and progress of treatment for patients with ITP. We will utilize the tests particularly for following patients pre- and post-treatment in a pilot study with NHLBI in the treatment setting of T-cell-depleted auto-stem cell transplantation in patients with severe ITP. Ten patients have been studied.